Liquid carbon dioxide absorbents, methods of using the same, and related systems

ABSTRACT

A carbon dioxide absorbent composition is described, including (i) a liquid, nonaqueous silicon-based material, functionalized with one or more groups that either reversibly react with CO 2  or have a high-affinity for CO 2 , and (ii) a hydroxy-containing solvent that is capable of dissolving both the silicon-based material and a reaction product of the silicon-based material and CO 2 . The absorbent may be utilized in methods to reduce carbon dioxide in an exhaust gas, and finds particular utility in power plants.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is Divisional Application of U.S. patentapplication Ser. No. 13/332,843 (and claims priority therefrom), filedon Dec. 21, 2011, which itself is a continuation-in-part of U.S. patentapplication Ser. No. 12/512,577, filed on Jul. 30, 2009, which is acontinuation-in-part of application Ser. No. 12/343,905, filed on Dec.24, 2008. The present application is also related to U.S. patentapplication Ser. No. 12/512,105, filed on Jul. 30, 2009; and now issuedas U.S. Pat. No. 8,030,509. The contents of the three Applications andthe issued patent are incorporated herein by reference, to the extentthey are consistent with the definitions utilized herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under grant numberDE-NT0005310 awarded by the Department of Energy-NETL. The Governmenthas certain rights in the invention.

BACKGROUND

Pulverized coal (PC) power plants currently produce over half theelectricity used in the United States. In 2007, these plants emittedover 1900 million metric tons of carbon dioxide (CO₂), and as such,accounted for 83% of the total CO₂ emissions from electric powergenerating plants and 33% of the total US CO₂ emissions. Eliminating, oreven reducing, these emissions will be essential in any plan to reducegreenhouse gas emissions.

Separating CO₂ from gas streams has been commercialized for decades infood production, natural gas sweetening, and other processes. Aqueousmonoethanolamine (MEA) based solvent capture is currently considered tobe the best commercially available technology to separate CO₂ fromexhaust gases, and is the benchmark against which future developments inthis area will be evaluated. Unfortunately, such amine-based systemswere not designed for processing the large volumes of flue gas producedby a PC plant. Scaling the MEA-based CO₂ capture system to the sizerequired for PC plants would result in an 83% increase in the overallcost of electricity for the PC plant. Applying this technology to allexisting PC plants in the US would cost $125 billion per year, makingMEA-based CO₂ capture an unlikely choice for large-scalecommercialization.

There are many properties that desirably would be exhibited, orenhanced, in any CO₂ capture technology contemplated to be a feasiblealternative to the currently utilized MEA-based systems. For example,any such technology would desirably exhibit a high net CO₂ capacity, andcould provide lower capital and operating costs (less material volumerequired to heat and cool, therefore less energy required). A lower heatof reaction would mean that less energy would be required to release theCO₂ from the material. Desirably, the technology would not require apre-capture gas compression so that a high net CO₂ capacity could beachieved at low CO₂ partial pressures, lowering the energy required forcapture.

Moreover, CO₂ capture technologies utilizing materials with lowerviscosities would provide improved mass transfer, reducing the size ofequipment needed, as well as a reduction in the cost of energy to runit. Low volatility and high thermal, chemical and hydrolytic stabilityof the material(s) employed could reduce the amount of material needingto be replenished, and could reduce the emission of degradationproducts. Of course, any such technology would also desirably have lowmaterial costs so that material make-up costs for the system would beminimized. Operability of CO₂ release at high pressures could reduce theenergy required for CO₂ compression prior to sequestration. Thesetechnologies would also desirably exhibit reduced corrosivity to helpreduce capital and maintenance costs, and further would not requiresignificant cooling to achieve the desired net CO₂ loading, reducingoperating costs.

In some cases, it would be very desirable if the new CO₂ capturetechnology could maintain reaction materials and reaction products in aliquid state on a relatively consistent basis. This would also allowbetter handling and transport of the materials through the CO₂ capturesystems, and could also contribute to lower operating costs.

Unfortunately, many of the above delineated desired properties interactand/or depend on one another, so that they cannot be variedindependently; and trade-offs are required. For example, in order tohave low volatility, the materials used in any such technology typicallymust have a fairly large molecular weight; but to have low viscosity,the materials must have a low molecular weight. To have a high CO₂capacity at low pressures, the overall heat of reaction needs to behigh; but to have low regeneration energy, the overall heat of reactionneeds to be low. Moreover, as of this time, it has been very difficult(if not impossible) to find CO₂ capture materials that have relativelyhigh CO₂ absorbance capabilities, but that can also remain in a liquidstate throughout the capture process.

Desirably, a CO₂ capture technology would be provided that optimizes asmany of the above desired properties as possible, yet without causingsubstantial detriment to other desired properties. At a minimum, inorder to be commercially viable, such technology would desirably be lowcost; and would utilize materials(s) having low volatility and highthermal stability. The materials should also have a high net capacityfor CO₂; and the capacity to remain in the liquid state during the CO₂capture process.

BRIEF DESCRIPTION

In a first aspect, there is provided a carbon dioxide absorbentcomprising (i) a liquid, nonaqueous silicon-based material,functionalized with one or more groups that reversibly react with CO₂and/or have a high-affinity for CO₂; and (ii) a hydroxy-containingsolvent that is capable of dissolving both the silicon-based materialand a reaction product of the silicon-based material and carbon dioxide

Also, a second aspect provides a method for reducing the amount ofcarbon dioxide in a process stream comprising contacting the stream witha carbon dioxide absorbent comprising (i) a liquid, nonaqueous,silicon-based material, functionalized with one or more groups thatreversibly react with CO₂ and/or have a high-affinity for CO₂, and (ii)a hydroxy-containing solvent, as mentioned above and further describedbelow.

In a third aspect, a power plant is provided, comprising a carbondioxide removal unit, and further comprising a carbon dioxide absorbent,as described herein.

A method of generating electricity with reduced carbon dioxide emissionsis also provided. The method comprises combusting a fuel (pulverizedcoal, liquid hydrocarbon, natural gas and the like), and directing theflue gas comprising carbon dioxide to an electricity-generating system,e.g. a steam or gas turbine, and then to a carbon dioxide removal unitcomprising a carbon dioxide absorbent, as described herein.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as is commonly understood by one of skill in theart to which this invention belongs. The terms “first”, “second”, andthe like, as used herein, do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.Also, the terms “a” and “an” do not denote a limitation of quantity, butrather denote the presence of at least one of the referenced items, andthe terms “front”, “back”, “bottom”, and/or “top”, unless otherwisenoted, are merely used for convenience of description, and are notlimited to any one position or spatial orientation.

If ranges are disclosed, the endpoints of all ranges directed to thesame component or property are inclusive and independently combinable(e.g., ranges of “up to about 25 wt. %”, or, more specifically, “about 5wt. % to about 20 wt. %”, is inclusive of the endpoints and allintermediate values of the ranges of “about 5 wt. % to about 25 wt. %,”etc.). The modifier “about” used in connection with a quantity isinclusive of the stated value, and has the meaning dictated by thecontext (e.g., it includes the degree of error associated withmeasurement of the particular quantity).

The subject matter disclosed herein relates generally to carbon dioxideabsorbents, power plants incorporating them, and methods of using theabsorbents to absorb carbon dioxide from process streams, e.g., as maybe produced by methods of generating electricity. Conventional carbondioxide absorbents lack one or more of the properties consideredimportant, if not critical, in the commercial feasibility of their usein many technologies. MEA-based aqueous absorbents, for example, werenot designed for use with large volumes of exhaust gas. As a result, useof these absorbents in such processes is extremely energy-intensive andcostly—too costly for implementation into power plants for postcombustion CO₂ capture. Moreover, the use of CO₂ absorbents and relatedreaction products that cannot remain in the liquid state throughout theCO₂ capture process would represent another problem for commercialfeasibility, as described herein.

Embodiments of the present invention are directed to carbon dioxideabsorbents comprising liquid, nonaqueous silicon-based materials, and ahydroxy-containing solvent. Silicon-based materials are defined asmolecules having between one and twenty repeat units, and thus, mayinclude small molecules comprising silicon, i.e., molecules comprisingfrom one to five silicon atoms, or oligomeric materials comprisingbetween about 5 and 20 silicon atoms.

In one embodiment, the present absorbent comprises a CO₂-philic,silicon-containing oligomer, e.g., comprising less than about 20repeating, monomeric units, and desirably from about 5 to about 10repeating monomeric units. As used herein, the term “CO₂-philic siliconcontaining oligomer” means an oligomer that has an affinity for CO₂, asmay be evidenced by solubility in liquid or supercritical CO₂, or anability to physically absorb CO₂

In some particular embodiments, the silicone materials are well-suitedfor use in the present absorbents. Also correctly referred to aspolymerized siloxanes or polysiloxanes, silicones are mixedinorganic-organic polymers or oligomers with the chemical formula[R₂SiO]_(n), wherein R comprises a linear, branched or aromatic organicgroup of any number of carbons, e.g., methyl ethyl, phenyl, etc. Thesematerials thus comprise an inorganic silicon-oxygen backbone ( . . .Si—O—Si—O—Si—O— . . . ) with organic side groups attached to the siliconatoms, which are four-coordinate. These silicones may be linear, with Rand OR′ end-capping groups; or may be cyclic in structure, containingonly the repeating units. An example of the latter isoctamethylcyclotetrasiloxane. Branched silicones can also be used.

Silicones have low volatility, even at short chain lengths, and in theliquid state at room temperature. They are typically low cost, andstable at high temperatures, e.g., up to about 150° C. Silicones arealso readily functionalized, and so, can be functionalized with groupsthat increase their affinity for CO₂.

The length of the silicone oligomer chain can be easily controlledduring synthesis, thereby allowing control of such physical propertiesas viscosity and boiling point. In addition, siloxane bonds arethermally stable and hydrolytically stable in the absence of strongacids or bases. Many silicone precursors are commercially available, andso advantageously, large scale production capabilities would not have tobe developed. Many of these may be utilized in the present invention.One example of a silicone suitable for functionalization in the presentinvention, and available from a variety of sources, comprisespolyhydridomethylsiloxane.

In another embodiment, the present absorbent comprises a CO₂-philic,silicon-based small molecule, e.g., comprising from about one to aboutfive silicon atoms. As used herein, the term “CO₂-philic silicon-basedsmall molecule” means a material that reversibly reacts with or has anaffinity for CO₂.

The silicon-based small molecules may comprise one silicon atom as shownin Formula (I), wherein L=a linking group of C₁-C₁₈, and may bealiphatic, aromatic, heteroaliphatic, heteroaromatic or mixturesthereof:

In formula (I), R₁, R₂, R₃ may be the same or different, and may beC₁-C₁₈ aliphatic, aromatic, heteroaliphatic, heteroaromatic or mixturesthereof and R₄=NR₅R₆ where at least one of R₅ or R₆ is H. The other maybe C₁-C₁₈ aliphatic, aromatic, heteroaliphatic, heteroaromatic ormixtures thereof.

In some embodiments, the silicon-based materials may be as shown inFormulas II-VI. When x≦5, y+z≦5 and/or r≦5, these materials wouldgenerally be considered to be silicon-based small molecules.Alternatively, when x≧5, y+z≧5 and/or r≧5, these silicon based materialswould generally be considered to be silicon-containing oligomers. Asdepicted in structures the core of the silicon-based small molecule maybe linear, cyclic, branched, or combinations of these configurations. Inmost embodiments related to these formulae, “x” and “y+z” each have avalue no greater than about 20; while the “r” value is usually nogreater than about 10.

For Formula II, R₇-R₁₂ may be the same or different. At least one ofR₇-R₁₂ will desirably be L-R₄, while the remainder are desirably C₁-C₁₈aliphatic, aromatic, heteroaliphatic, heteroaromatic, or mixturesthereof.

For formula III, R₁₃-R₁₆ may be the same or different. At least one ofR₁₃-R₁₆ will desirably be L-R₄, while the remainder are desirably C₁-C₁₈aliphatic, aromatic, heteroaliphatic, heteroaromatic or mixturesthereof. R₁₆ is SiRR′R″, wherein R, R′ and R″ may be the same ordifferent, and may be C₁-C₁₈ aliphatic, aromatic, heteroaliphatic,heteroaromatic or mixtures thereof, and may be L-R₄.

For formulae IV, V, and VI, R₁₈-R₂₃ and R₂₄-R₂₅ and R₂₆-R₃₅ may be thesame or different; and at least one of R₁₈-R₂₃=L-R₄; at least one ofR₂₄-R₂₅=L-R₄; and at least one of R₂₆-R₃₅=L-R₄, and the rest may beC₁-C₁₈ aliphatic, aromatic, heteroaliphatic, heteroaromatic, or mixturesthereof. A related proviso is that R₂₃ is SiRR′R″, wherein R, R′ and R″may be the same or different, and may be C₁-C₁₈ aliphatic, aromatic,heteroaliphatic, heteroaromatic or mixtures thereof, and may be L-R₄.

The silicon-based material may desirably be functionalized with groupsthat enhance its net capacity for CO₂. Functional groups that areexpected to be CO₂-philic, and react with CO₂ in a silicon-basedmaterial they functionalize, include any of those including a nitrogenatom, such as, for example aliphatic amines, imines, amidines, amides,heterocyclic amino compounds such as pyridine, aromatic amines such asaniline, and the like, as well as combinations of any of these. Theparticular functional group utilized will depend upon the silicon-basedmaterial chosen. For those embodiments wherein the silicon-basedmaterial comprises a siloxane, amine functionality may be suitable,since many aminosiloxanes are readily commercially available, and arereadily further functionalized if desired or required, in order toincrease CO₂ reactivity. Non-limiting examples of amine functionalgroups that exhibit CO₂-reactivity include aminomethyl, aminoethyl,aminopropyl, aminoethyl-aminopropyl, aminoethyl-aminoisobutyl,aminoethylaminomethyl, 2-aminopyridyl, piperazine-propyl and imidazoylpropyl.

Functional groups may be located in a side chain, and can also be theend-capping groups. Aminoethyl-aminopropyl siloxane oligomers withfunctional groups in the side chain are exemplified by the moleculeshown below at FIGURE VII. This material has a maximum theoretical CO₂capacity of about 20 wt %, compared to 10 wt % for 30 wt % aqueous MEA.

One other example of an aminosiloxane with end-capped functional groupssuitable for use in the present absorbent is aminopropyl terminatedpolydimethyldisiloxane, shown below in FIGURE VIII:

One such aminosiloxane is used for hair conditioning, and iscommercially available from Gelest, with a number average molecularweight of from about 850 to about 900, and a calculated CO₂ absorptioncapacity of from about 4.4 to about 5.2%. It is expected that theaddition of further amine functionality will result in an increase inthis absorption capacity.

Those of ordinary skill in the art of polymer chemistry are well versedin methods of adding functional groups to the backbone of an oligomeruseful in the present absorbent. Numerous methods of attachment offunctional groups are known such as hydrosilylation and displacement asshown in Michael A. Brook's book Silicon in Organic, Organometallic, andPolymer Chemistry (Wiley VCH Press, 2000).

As alluded to previously, in many cases, silicon-based functionalizedmaterials form solids or very high viscosity oils on reaction with CO₂.This can negatively impact mass transfer, so that the absorbent materialdoes not react with as much CO₂ as is theoretically possible.Furthermore, materials that form solid CO₂ reaction products would notreadily fit into existing CO₂ capture process schemes. Therefore, aselected co-solvent is added according to embodiments of this invention.The co-solvent is hydroxy-containing, as explained below, and maintainsliquidity on reaction with CO₂, thereby maximizing capture efficiency,and enhancing the compatibility of the selected materials with standardCO₂ capture systems.

The concept of using co-solvents in conjunction with aminoalkanols toabsorb CO₂ from mixed gas streams was generally known in the art.Reference is made, for example, to U.S. Pat. No. 4,112,051, issued toSartori et al. However, the success of this approach with materials thatare much less polar (like many of the silicon-based materials of thepresent invention) might appear doubtful to those skilled in the art. Inorder to fulfill its function of maintaining solution liquidity, theco-solvent must be very miscible with both the silicon-based materialand its CO₂ reaction product. In view of the difference in polaritybetween typical siloxane materials and hydroxy-containing solvents,identification of suitable solvents was a very difficult task.

Surprisingly, the present inventors identified certainhydroxy-containing solvents that are able to solubilize both thesilicon-based materials described above, and their CO₂ reactionproducts. As used herein, the phrase “hydroxy-containing solvent” meansa solvent that has one or more hydroxy groups. The hydroxy-containingsolvent also desirably has a low vapor pressure, e.g., below about 150mm Hg at 100° C.; and often, from about 0.001 to about 30 mm Hg at 100°C., so that minimal loss of the hydroxy-containing solvent occurs viaevaporation.

Suitable hydroxy-containing solvents for embodiments of this inventionare liquid at room temperature, and are capable of dissolving thesilicon-based material and its CO₂ reaction product. In preferredembodiments, the solvents are capable of dissolving the constituents atrelatively high concentrations, e.g., at least about 20% of thesilicon-based material, and in some preferred embodiments, at leastabout 40% of the silicon-based materials. In this manner, a stablesolution of the silicon-based material and a carbamic-acid salt (one ofthe typical reaction products) are present as a stable solution.

In preferred embodiments, the hydroxy-containing solvents do not,themselves, substantially chemically react with CO₂. Rather, they serveas a medium for CO₂ transfer to the functionalized silicon-basedmaterial. As a result, the hydroxy-containing solvents are expected tobe capable of increasing the reaction rate, e.g., by increasing the masstransfer rate, of CO₂ and the silicon-based material, and also toreduce, or substantially prevent, excessive viscosity build-up when thesilicon-based material reacts with CO₂. Advantageously, many suitablehydroxyl-containing solvents may be recycled, along with thesilicon-based material, if desired.

Examples of suitable hydroxy-containing solvents include, but are notlimited to, those comprising one or more hydroxyl groups, such as,glycols, phenols, and hydroxylated silicones. Suitable glycols mayinclude, for example, trimethylolpropane ethoxylates, glycerol, ethyleneglycol, diethylene glycol, triethylene glycol and tetraethylene glycol,to name a few. Suitable hydroxylated silicones include, for example,1,3-bis(3-hydroxypropyl)tetramethyldisiloxane, or the hydrosilylationreaction product of 1,1,3,3-tetramethyldisiloxane and trimethylolpropaneallylether. Hydroxy compounds may also be in the form of phenols such aseugenol, isoeugenol, 2-allyl-6-methylphenol, 2-allylphenol and the like.

In certain embodiments, the absorbent may comprise an amount of water,e.g., so that all water need not be removed from the process stream inorder to utilize the absorbent and methods. Indeed, in some embodiments,water is desirably present and in such embodiments, can assist in thesolubilization of reaction products.

Optionally, the absorbent may also include other components, such as,e.g., oxidation inhibitors to increase the oxidative stability andanti-foaming agents. The use of oxidation inhibitors, also calledantioxidants, can be especially advantageous in those embodiments of theinvention wherein the functional groups comprise amine groups.

The carbon dioxide absorbents provided herein are expected to providesubstantial improvement when utilized to remove CO₂ from process gases,as compared to those currently commercially available and/or utilizedfor this purpose. As such, a method of reducing the carbon dioxide in aprocess stream is provided, and comprises contacting the process streamwith the carbon dioxide absorbents described herein. The process streamso treated may be any wherein the level of CO₂ therein is desirablyreduced, and in many processes, CO₂ is desirably reduced at least in theexhaust streams produced thereby. The process stream is typicallygaseous, but may contain solid or liquid particulates, and may be at awide range of temperatures and pressures, depending on the application.

The carbon dioxide absorbents for embodiments of this invention have lowvolatility, high thermal stability, and are either commerciallyavailable with, or can be provided with, a high net capacity for CO₂,and as such, are appropriate for large scale implementation. And so,there is also provided a power plant utilizing the present absorbents,and a method of utilizing the absorbents in generating electricity withreduced carbon dioxide emissions.

Examples 1-12 Reaction of Silicon-Based Materials with CO₂ in thePresence of a Hydroxy-Containing Co-Solvent

To illustrate the ability of the hydroxy-containing co-solventtriethylene glycol to enhance the CO₂ absorption of varioussilicon-based materials, as well as providing a liquid medium, thefollowing Examples 1-12 were conducted. The silicon-based materials wereexposed to 1 atmosphere of CO₂ in the presence of, or not in thepresence of, the hydroxyl-containing co-solvent triethylene glycol (at50 wt %, with the exception of example 4 at 75 wt %) at 40° C. for 2hours (h) with mechanical stirring.

Comparative Example 1

Into a pre-tared, 25 mL, three-neck, round-bottom flask equipped with amechanical stirrer, gas inlet and a gas outlet and heated with atemperature controlled oil bath, was charged 2.0707 g of1,3-Bis(3-aminopropyl)tetramethyldisiloxane. Dry CO₂ gas was introducedat a rate of ˜50 mL/min into the flask via a glass tube positioned ˜10mm above the stirring liquid surface. CO₂ exposure continued for 2-h at40° C. after which time the exterior of the flask was cleaned and theflask weighed. The total weight gain of 0.3588 g corresponded to 71% ofthe theoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was also a solid.

Example 2

2.0194 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 2.0174 g oftriethylene glycol were charged into a flask and allowed to react withCO₂ according to the procedure described in Example 1. The total weightgain was 0.4089 g. This corresponded to 114% of the theoretical amountof weight that should have been gained if all the amine groups hadreacted with a stoichiometric amount of CO₂. In contrast to Example 1,the reaction product here was a liquid.

Comparative Example 3

2.0653 g of aminoethylaminopropyl methylsiloxane oligomer were chargedinto a flask and allowed to react with CO₂ according to the proceduredescribed in Example 1. The total weight gain was 0.2110 g. Thiscorresponded to 37% of the theoretical amount of weight that should havebeen gained if all the amine groups had reacted with a stoichiometricamount of CO₂. The reaction product was a solid.

Example 4

2.0168 g of aminoethylaminopropyl methylsiloxane oligomer and 4.0292 gof triethylene glycol were charged into a flask and allowed to reactwith CO₂ according to the procedure described in Example 1. The totalweight gain was 0.4803 g. This corresponded to 87% of the theoreticalamount of weight that should have been gained if all the amine groupshad reacted with a stoichiometric amount of CO₂. The reaction productwas a liquid, in contrast to comparative Example 3, which did notinclude triethyleneglycol.

Comparative Example 5

2.0295 g of 1,3-Bis(3-aminoethylaminopropyl)-tetramethyldisiloxane werecharged into a flask and allowed to react with CO₂ according to theprocedure described in Example 1. The total weight gain was 0.3389 g.This corresponded to 64% of the theoretical amount of weight that shouldhave been gained if all the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a solid.

Example 6

2.0240 g of 1,3-Bis(3-aminoethylaminopropyl)-tetramethyldisiloxane and2.0237 g of triethylene glycol were charged into a flask and allowed toreact with CO₂ according to the procedure described in Example 1. Thetotal weight gain was 0.4777 g. This corresponded to 90% of thetheoretical amount of weight that should have been gained if all of theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a liquid, in contrast to the solid obtained inexample 5.

Comparative Example 7

1.1090 g of1,3,5,7-tetrakis(3-aminopropyl)-tetramethylcyclotetrasiloxane werecharged into a flask and allowed to react with CO₂ according to theprocedure described in Example 1. The total weight gain was 0.0621. Thiscorresponded to 30% of the theoretical amount of weight that should havebeen gained if all the amine groups had reacted with a stoichiometricamount of CO₂. The reaction product was a solid.

Example 8

1.0722 g of1,3,5,7-tetrakis(3-aminopropyl)-tetramethylcyclotetrasiloxane and 1.1028g of triethylene glycol were charged into a flask and allowed to reactwith CO₂ according to the procedure described in Example 1 The totalweight gain was 0.3099 g. This corresponded to 154% of the theoreticalamount of weight that should have been gained if all the amine groupshad reacted with a stoichiometric amount of CO₂. The reaction productwas a liquid, in contrast to the solid product obtained in example 7.

Comparative Example 9

1.0498 g of Tetrakis(3-aminopropyl-dimethylsiloxy)silane were chargedinto a flask and allowed to react with CO₂ according to the proceduredescribed in Example 1. The total weight gain was 0.1445 g. Thiscorresponded to 87% of the theoretical amount of weight that should havebeen gained if all the amine groups had reacted with a stoichiometricamount of CO₂. The reaction product was a solid.

Example 10

1.0662 g of Tetrakis(3-aminopropyldimethylsiloxy)silane and 0.1.1175 gof triethylene glycol were charged into a flask and allowed to reactwith CO₂ according to the procedure described in Example 1. The totalweight gain was 0.1956 g. This corresponded to 116% of the theoreticalamount of weight that should have been gained if all the amine groupshad reacted with a stoichiometric amount of CO₂. The reaction productwas a liquid, in contrast to the solid obtained in example 9.

Comparative Example 11

1.2135 g of 1,3-Bis(3,9-dimethyl-5,8,11-trioxa-2-azatetradecan-13-amine)were charged into a flask and allowed to react with CO₂ according to theprocedure described in Example 1. The total weight gain was 0.0742 g.This corresponded to 44% of the theoretical amount of weight that shouldhave been gained if all the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a solid.

Example 12

1.0323 g of 1,3-Bis(3,9-dimethyl-5,8,11-trioxa-2-azatetradecan-13-amine)and 1.0368 g of triethylene glycol were charged into a flask and allowedto react with CO₂ according to the procedure described in Example 1. Thetotal weight gain was 0.0587 g. This corresponded to 41% of thetheoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a liquid, in contrast to the solid obtained inexample 11.

The results of Examples 1-12 are summarized in Table 1, below.

TABLE 1 Co- % of Physical solvent Theoretical state of Example Aminepresent wt gain product 1 1,3-Bis(3-aminopropyl)tetramethyldisiloxane No71 S comparative 2 1,3-Bis(3-aminopropyl)tetramethyldisiloxane Yes 114 L3 Aminoethylaminopropyl methylsiloxane No 37 S comparative oligomer 4Aminoethylaminopropyl methylsiloxane Yes 87 L oligomer 5 1,3-Bis(3- No64 S comparative aminoethylaminopropyl)tetramethyldisiloxane 61,3-Bis(3- Yes 90 L aminoethylaminopropyl)tetramethyldisiloxane 71,3,5,7-tetrakis(3- No 30 S comparativeaminopropyl)tetramethylcyclotetrasiloxane 8 1,3,5,7-tetrakis(3- Yes 154L aminopropyl)tetramethylcyclotetrasiloxane 9Tetrakis(3-aminopropyldimethylsiloxy)silane No 87 S comparative 10Tetrakis(3-aminopropyldimethylsiloxy)silane Yes 116 L 111,3-Bis(3,9-dimethyl-5,8,11-trioxa-2- No 44 S comparativeazatetradecan-13-amine) 12 1,3-Bis(3,9-dimethyl-5,8,11-trioxa-2- Yes 41L azatetradecan-13-amine)

Examples 13-20 Reaction of a Silicon-Based Material with CO₂ in thePresence of Various Hydroxy-Containing Co-Solvents

To illustrate the ability of the hydroxy-containing co-solventstriethyleneglycol dimethyl ether and triethyleneglycol to enhance theCO₂ absorption of the silicon-based material,1,3-bis(3-aminopropyl)tetramethyldisiloxane, as well as to provide aliquid medium, the following Examples 13-20 were conducted. In each, thesilicon-based material 1,3-bis(3-aminopropyl)tetramethyldisiloxane wasexposed to 1 atmosphere of CO₂ in the presence of or not in the presenceof a hydroxyl-containing co-solvent at 40° C. for 2 hours (h), withmechanical stirring.

Comparative Example 13

Into a pre-tared, 25 mL, three-neck, round-bottom flask equipped with amechanical stirrer, gas inlet and a gas outlet and heated with atemperature controlled oil bath, was charged 2.0707 g of1,3-Bis(3-aminopropyl)tetramethyldisiloxane. Dry CO₂ gas was introducedat a rate of ˜50 mL/min into the flask via a glass tube positioned ˜10mm above the stirring liquid surface. CO₂ exposure continued for 2 h at40° C. after which time the exterior of the flask was cleaned and theflask weighed. The total weight gain of 0.3588 g corresponded to 71% ofthe theoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was also a solid.

Example 14

2.0261 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 2.1198 g oftriethyleneglycol dimethyl ether were charged into a flask and allowedto react with CO₂ according to the procedure described in Example 13.The total weight gain was 0.2984 g. This corresponded to 83% of thetheoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a solid, which contrasts with the liquid productobtained in example 2, using triethylene glycol.

Example 15

2.0366 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 4.0306 g oftriethyleneglycol dimethyl ether were charged into a flask and allowedto react with CO₂ according to the procedure described in Example 13.The total weight gain was 0.3566 g. This corresponded to 99% of thetheoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a solid, which contrasts with the liquid productobtained in example 2, using triethylene glycol.

Example 16

2.0194 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 2.0174 g oftriethyleneglycol were charged into a flask and allowed to react withCO₂ according to the procedure described in Example 13. The total weightgain was 0.4089 g. This corresponded to 114% of the theoretical amountof weight that should have been gained if all the amine groups hadreacted with a stoichiometric amount of CO₂. The reaction product was aliquid.

Example 17

2.0230 g of triethyleneglycol was charged into a flask and allowed toreact with CO₂ according to the procedure described in Example 13. Thetotal weight gain was 0.0004 g. This corresponded to <1% total weightgain. The reaction product in this instance was a liquid, but exhibitedrelatively poor CO₂ absorbing-capacity for triethylene glycol. Thisexample demonstrates the necessity for having the silicon-based compoundpresent.

Example 18

2.0387 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 1.0454 g oftriethyleneglycol were charged into a flask and allowed to react withCO₂ according to the procedure described in Example 13. The total weightgain was 0.4071 g. This corresponded to 113% of the theoretical amountof weight that should have been gained if all the amine groups hadreacted with a stoichiometric amount of CO₂. The reaction product was asolid, in contrast to the reaction product of Example 2. This exampledemonstrates that, in some instances, there is an optimum ratio ofsilicone to solvent.

Example 19

2.0178 g of 1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 4.0734 g oftriethyleneglycol were charged into a flask and allowed to react withCO₂ according to the procedure described in Example 13. The total weightgain was 0.4203 g. This corresponded to 118% of the theoretical amountof weight that should have been gained if all the amine groups hadreacted with a stoichiometric amount of CO₂. The reaction product was aliquid.

Example 20

2.0186 g of 1,3-Bis(3-aminopropyl)-tetramethyldisiloxane, 1.0419 g oftriethyleneglycol and 0.2245 g water were charged into a flask andallowed to react with CO₂ according to the procedure described inExample 13. The total weight gain was 0.3545 g. This corresponded to 99%of the theoretical amount of weight that should have been gained if allthe amine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a liquid in contrast to Example 18. The exampledemonstrates that, in some instances, the use of additional water canfacilitate the formation of a liquid product.

The results of Examples 13-20 are summarized in Table 2, below.

TABLE 2 Wt ratios CO₂ uptake (Amine: (% Physical Example Co-solventsolvent:water) theoretical) state 13 (control) None 100:0:0 71 S 14Triethyleneglycol 50:50:0 83 S dimethyl ether 15 Triethyleneglycol33:67:0 99 S dimethyl ether 16 Triethyleneglycol 50:50:0 114 L 17Triethyleneglycol 0:100:0 — L 18 Triethyleneglycol 67:33:0 113 ~S 19Triethyleneglycol 33:67:0 118 L 20 Triethyleneglycol 62:31:07 99 L

Table 2 demonstrates that, without a co-solvent,1,3-bis(3-aminopropyl)tetramethyldisiloxane readily forms a solidmaterial (Example 13). When triethyleneglycol dimethyl ether is added asa co-solvent (Examples 14, 15), solid reaction products are stillformed. When triethyleneglycol is added at a 1:1 weight ratio (Example16) a homogeneous reaction product is formed that, very desirably,remains liquid throughout the capture process. Varying the ratio ofco-solvent to capture solvent results in varying degrees of liquidityand viscosity. (Examples 17-20)

Table 2 further demonstrates that the co-solvent alone does notphysically absorb a significant amount of CO₂ (Example 17). However, the“mixed system” allows for enhanced capture of CO₂ via a synergisticaction of the chemical capture process and physisorption. Water mayoptionally be present to aid in solubilizing the reaction products(Example 20).

Examples 21-33 Reaction of a Silicon-Based Material with CO₂ in thePresence of Various Hydroxy-Containing Co-Solvents

To illustrate the ability of various other hydroxy-containingco-solvents (at 50 weight % concentration) to enhance the CO₂ absorptionof the silicon-based material,1,3-bis(3-aminopropyl)tetramethyldisiloxane, as well as to provide aliquid medium, the following Examples 21-33 were conducted. In eachexample, the silicon-based material1,3-bis(3-aminopropyl)tetramethyldisiloxane was exposed to 1 atmosphereof CO₂ in the presence of, or not in the presence of, ahydroxyl-containing co-solvent at 40° C. for 2 hours (h) with mechanicalstirring.

Comparative Example 21

Into a pre-tared, 25 mL, three-neck, round-bottom flask equipped with amechanical stirrer, gas inlet and a gas outlet, and heated with atemperature controlled oil bath, were charged 2.0349 g of1,3-Bis(3-aminopropyl)tetramethyldisiloxane and 2.0472 g of SF1488 (asilicone polyether available from Momentive Performance Materials). DryCO₂ gas was introduced at a rate of ˜50 mL/min into the flask, via aglass tube positioned ˜10 mm above the stirring liquid surface. CO₂exposure continued for 2 h at 40° C., after which time the exterior ofthe flask was cleaned, and the flask weighed. The total weight gain of0.2739 g corresponded to 76% of the theoretical amount of weight thatshould have been gained if all of the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a waxy yellowsolid.

Example 22

2.0311 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0506 gof a diol terminated disiloxane prepared via the hydrosilylation oftrimethylolpropane mono allyl ether with tetramethyl disiloxane werecharged into a flask and allowed to react with CO₂, according to theprocedure described in Example 21. The total weight gain was 0.3491 g.This corresponded to 97% of the theoretical amount of weight that shouldhave been gained if all the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a viscous liquid.

Example 23

2.0337 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0653 gof tetrathylene glycol were charged into a flask and allowed to reactwith CO₂ according to the procedure described in Example 21. The totalweight gain was 0.4182 g. This corresponded to 116% of the theoreticalamount of weight that should have been gained if all the amine groupshad reacted with a stoichiometric amount of CO₂. The reaction productwas a moderately viscous liquid.

Example 24

2.0745 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0507 gof poly(propylene glycol) with a molecular weight of 725 were chargedinto a flask and allowed to react with CO₂ according to the proceduredescribed in Example 21. The total weight gain was 0.3400 g. Thiscorresponded to 92.5% of the theoretical amount of weight that shouldhave been gained if all the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a solid thatformed very quickly on introduction of the gas.

Example 25

1.9957 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0630 gof a 3:1 blend of tetraethylene glycol and the diol terminateddisiloxane from example B above, were charged into a flask and allowedto react with CO₂ according to the procedure described in Example 21.The total weight gain was 0.3937 g. This corresponded to 111% of thetheoretical amount of weight that should have been gained if all theamine groups had reacted with a stoichiometric amount of CO₂. Thereaction product was a moderately viscous liquid.

Example 26

2.0385 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 1.9718 gof trimethyolpropane mono allyl ether were charged into a flask andallowed to react with CO₂ according to the procedure described inExample 21. The total weight gain was 0.3876 g. This corresponded to107% of the theoretical amount of weight that should have been gained ifall the amine groups had reacted with a stoichiometric amount of CO₂.The reaction product was a moderately viscous liquid.

Example 27

2.0683 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0709 gof eugenol were charged into a flask and allowed to react with CO₂according to the procedure described in Example 21. The total weightgain was 0.3449 g. This corresponded to 94% of the theoretical amount ofweight that should have been gained if all the amine groups had reactedwith a stoichiometric amount of CO₂. The reaction product was a viscousyellow liquid.

Example 28

2.0291 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 1.9965 gof trimethylolpropane ethoxylate (4/15 EO/OH, Mn ˜170) were charged intoa flask and allowed to react with CO₂ according to the proceduredescribed in Example 21. The total weight gain was 0.3640 g. Thiscorresponded to 101% of the theoretical amount of weight that shouldhave been gained if all the amine groups had reacted with astoichiometric amount of CO₂. The reaction product was a very viscousyellow liquid.

Example 29

2.0157 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0675 gof pentaerythritol ethoxylate (3/4 EO/OH, Mn ˜270) were charged into aflask and allowed to react with CO₂ according to the procedure describedin Example 21. The total weight gain was 0.3855 g. This corresponded to108% of the theoretical amount of weight that should have been gained ifall the amine groups had reacted with a stoichiometric amount of CO₂.The reaction product was a very viscous liquid.

Example 30

1.9999 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0129 gof a 2:1 blend of tetraethylene glycol and eugenol were charged into aflask and allowed to react with CO₂ according to the procedure describedin Example 21. The total weight gain was 0.3773 g. This corresponded to107% of the theoretical amount of weight that should have been gained ifall the amine groups had reacted with a stoichiometric amount of CO₂.The reaction product was a viscous yellow liquid.

Example 31

2.0391 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0640 gof isoeugenol were charged into a flask and allowed to react with CO₂according to the procedure described in Example 21. The total weightgain was 0.3464 g. This corresponded to 96% of the theoretical amount ofweight that should have been gained if all the amine groups had reactedwith a stoichiometric amount of CO₂. The reaction product was a viscousyellow liquid.

Example 32

2.0379 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0447 gof sulfolane were charged into a flask and allowed to react with CO₂according to the procedure described in Example 21. The total weightgain was 0.3227 g. This corresponded to 89% of the theoretical amount ofweight that should have been gained if all the amine groups had reactedwith a stoichiometric amount of CO₂. The reaction product was a solidthat formed very quickly on introduction of the gas.

Example 33

2.0118 g of 1,3-Bis(3-aminopropyl) tetramethyl-disiloxane and 2.0010 gof 2-allyl-6-methylphenol were charged into a flask and allowed to reactwith CO₂ according to the procedure described in Example 21. The totalweight gain was 0.2711 g. This corresponded to 76% of the theoreticalamount of weight that should have been gained if all the amine groupshad reacted with a stoichiometric amount of CO₂. The reaction productwas a light yellow liquid.

Example 33A

1.0258 g of 1,3-Bis(3-aminopropyl)-tetramethyldisiloxane and 1.0266 gglycerol were charged to a flask and allowed to react with CO₂,according to the procedure described above for Example 1. The totalweight gain was 0.1888 g. This corresponds to 104% of the theoreticalamount of weight that should have been gained if all the amine groupsreacted with a stoichiometric amount of CO₂. The final mixture was aliquid.

The results of Examples 21-33 and 33A are summarized in Table 3, below.

TABLE 3 % of Physical Theoretical state of Example Co-Solvent (50 wt %)wt gain product 21 SF1488 Momentive Silicone polyether 76 S Comparative22 a hydrosilylation reaction product of 1,1,3,3- 97 Ltetramethyldisiloxane and trimethylolpropane allyl ether 23Tetraethylene glycol 116 L 24 Poly(propylene glycol) MW = 725 93 S 25Mixture of tetraethylene glycol/1,3-bis(3- 111 Lhydroxypropyl)tetramethyldisiloxane 3:1 26 Trimethylolpropane allylether 107 L 27 Eugenol 94 L 28 Trimethylolpropane ethoxylate Mn = 170101 L 29 Pentaerythritol ethoxylate Mn = 270 108 L 30 2:1 Tetraethyleneglycol/eugenol 107 L 31 Isoeugenol 96 L 32 Sulfolane 89 S comparative 332-allyl-6-methylphenol 76 L 33A Glycerol 104 L

Examples 34-50 High Throughput Screening Experiments

The high throughput screening experiments were carried out using a 27well parallel reactor (React Vap III) from Pierce and a Symyx CoreModule for automated weighing in 8 mL glass vials. The experiments wererun using technical grade CO₂ at 1 atm and the flow was set at 1.2 mL/h(10000 cm²/min) by using a MKS gas flow controller. Each formulation wastested in triplicate. The co-solvents were purchased from Aldrich orFisher Scientific and used without further purification.

Each vial was loaded with a stirrer bar and pre-weighed using the SymyxCore module. The vials were then loaded with the corresponding compound(200-300 μL) and the appropriate co solvent (200-300 μL). The resultingmixture was stirred for 15-20 min and treated with CO₂ gas (1 atm) for60-120 min at the desired temperature (40 and 55° C.). After the CO₂treatment, the reactor block was cooled down to room temperature and allthe vials were transferred to a Symyx Core Module® for automatedweighing. The physical state of each vial was visually inspected andrecorded. The CO₂ adsorption performance was reported as an average ofthe % weight gain after each CO₂ treatment. The results of theseexperiments are shown in Table 4.

TABLE 4 % wt Physical Ex. Silicon-based Material Co-Solvent Wt ratios*%** gain state*** 34 1,3-Bis(3-aminopropyl) triethylene 10:90 175 3.1 Ltetramethyldisiloxane glycol 35 1,3-Bis(3-aminopropyl) triethylene 30:70155 8.2 L tetramethyldisiloxane glycol 36 1,3-Bis(3-aminopropyl)triethylene 50:50 133 11.8 L tetramethyldisiloxane glycol 371,3-Bis(3-aminopropyl) N methyl 50:50 118 10.4 L tetramethyldisiloxanepyrrolidone (NMP) 38 1,3-Bis(3-aminopropyl) Tetraglyme 50:50 123 10.9 SComp tetramethyldisiloxane dimethyl ether 39 Aminoethylaminopropyltriethylene 10:90 165 4.6 L methylsiloxane oligomers glycol 40Aminoethylaminopropyl triethylene 30:70 122 10.2 L methylsiloxaneoligomers glycol 41 Aminoethylaminopropyl triethylene 50:50 44 6.2 Lmethylsiloxane oligomers glycol 42 Aminoethylaminopropyl N methyl 50:5056 7.8 S Comp methylsiloxane oligomers pyrrolidone 43Aminoethylaminopropyl Tetraglyme 50:50 41 5.7 S Comp methylsiloxaneoligomers dimethyl ether 44 1,3,5,7-tetrakis(3- N methyl 50:50 84 7.9 SComp aminopropyl) pyrrolidone tetramethylcyclotetrasiloxane 451,3,5,7-tetrakis(3- Tetraglyme 50:50 53 5.0 S Comp aminopropyl) dimethyltetramethylcyclotetrasiloxane ether 46 1,3- triethylene 10:90 158 5.0 LBis(aminoethylaminomethyl) glycol tetramethyldisiloxane 47 1,3-triethylene 30:70 117 11.1 L Bis(aminoethylaminomethyl) glycoltetramethyldisiloxane 48 1,3- triethylene 50:50 74 11.7 LBis(aminoethylaminomethyl) glycol tetramethyldisiloxane 49 1,3- N methyl50:50 59 9.3 S Comp Bis(aminoethylaminomethyl) pyrrolidonetetramethyldisiloxane 50 1,3- Tetraglyme 50:50 64 10.1 S CompBis(aminoethylaminomethyl) dimethyl tetramethyldisiloxane ether*Amine:solvent **of theoretical ***After absorption

Generally speaking, Table 4 shows that capped-hydroxy compounds yieldedsolid reaction products, while uncapped co-solvents yielded solublesolutions. The amide-based compound, N-methyl pyrrolidone (NMP), did notperform successfully as a solvent.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method for reducing the amount of carbon dioxide in a processstream comprising contacting the stream with a carbon dioxide absorbentcomprising (i) a liquid, nonaqueous silicon-based material,functionalized with one or more groups that reversibly react with CO₂and/or have a high-affinity for CO₂, and (ii) a hydroxy-containingsolvent that is capable of dissolving both the silicon-based materialand a reaction product of the silicon-based material and carbon dioxide.2. The method of claim 1, wherein the process stream comprises anexhaust stream.
 3. The method of claim 1, wherein the hydroxy-containingsolvent comprises trimethylolpropane ethoxylates, glycerol, ethyleneglycol, diethylene glycol, triethylene glycol, tetraethyl ene glycol,1,3-bis(3-hydroxypropyl)tetramethyldisiloxane, a hydrosilylationreaction product of 1,1,3,3-tetramethyldisiloxane and trimethylolpropaneallylether, or combinations of these.
 4. The method of claim 1, whereinthe carbon dioxide solvent further comprises water.
 5. The method ofclaim 1, wherein the functionalized silicon-based material comprises oneor more aminosilicones.
 6. The method of claim 1, wherein the functionalgroup(s) comprise(s) one or more amines.
 7. The method of claim 6,wherein the functional group(s) comprise(s) one or more di-, tri- andpolyamines, or combinations of the forgoing.
 8. A power plant comprisinga carbon dioxide removal unit that further includes a carbon dioxideabsorbent comprising (i) a liquid, nonaqueous silicon-based material,functionalized with one or more groups that reversibly react with CO₂and/or have a high-affinity for CO₂; and (ii) a hydroxy-containingsolvent that is capable of dissolving both the silicon-based materialand a reaction product of the silicon-based material and CO₂.